Implantable medical devices (IMDs) are implantable or partially implantable. Some examples of IMDs include cardiac function management (CFM) devices such as implantable pacemakers, implantable cardioverter defibrillators (ICDs), cardiac resynchronization therapy devices (CRTs), and devices that include a combination of such capabilities. The devices can be used to treat patients or subjects using electrical or other therapy, or to aid a physician or caregiver in patient diagnosis through internal monitoring of a patient's condition. The devices may include one or more electrodes in communication with one or more sense amplifiers to monitor electrical heart activity within a patient, and often include one or more sensors to monitor one or more other internal patient parameters. The devices can be implanted subcutaneously and can include electrodes that are able to sense cardiac signals without being in direct contact with the patient's heart. Other examples of IMDs include implantable diagnostic devices, implantable drug delivery systems, or implantable devices with neural stimulation capability (e.g., vagus nerve stimulator, baroreflex stimulator, carotid sinus stimulator, deep brain stimulator, sacral nerve stimulator, etc.).
IMDs can include circuitry mounted within a hermetically-sealed device housing. The circuitry can be operatively connected to a lead which is implanted on or in the heart. The circuitry can be used for, for example, generating electrical signals that are delivered to the patient's heart through one or more feedthrough conductors that pass from the interior of the device housing to the exterior of the device housing. The feedthrough assembly provides a mechanism for electrical signal transfer through the hermetically sealed device housing. This hermetic seal serves to isolate the circuitry within the metal case from tissue, blood, and other patient fluid.
In addition to the electrical signals generated by the circuitry of the IMD, externally generated electromagnetic signals can also pass through the hermetic seal via the feedthrough assembly and interfere with proper operation of the implantable medical device. Thus, electromagnetic interference filters can be integrated into IMDs to filter these externally generated electromagnetic signals to maintain the intended voltage levels along the feedthrough conductors.